Reticulated webs and method and apparatus for their production



Oct. 22, 1957 D. E. TILL ETAL RETICULATED WEBS AND METHOD ANDAPPARATUS FOR THEIR PRODUCTION Filed Dec. 24, 1953 ited States Patent O RETICULATED WEBSAND METHOD AND APPARATUS FOR THER PRGDUCTION Derek E.Till, Concord, and .CarlRz-Smallman, Wollaston, Mass, assignors, :by mesne;assignmen'is, to American Viscose Corporation, Philadelphia, Pa., a-corporation of Delaware Application December24, 1953, Serial No. 400,252

20 Claims. -(Cl...-15427) This invention relates to fibrous bodies and more particularly to reticulated webs or structures formed of fibrous materials and to methods and apparatus for preparing such bodies or structures.

Fibrous bodies or structures of:fiber-forming organic substances are generally .formed from filaments prepared by either extruding the fiber-forming organic substance through an orifice or by spraying the organic substance by the use of spray guns and the like. Methods which involve the extrusion of the fiber-forming substance through orifices are limited to the use of certain substances which may be filtered and which possess certain other characteristics, including wet and dry tensile strengths, necessary for the spinning operations. The filaments produced by conventional extrusion methods are continuousfilaments and the'size .or denier is limited by the extrusion characteristics of the substancs or of the spinning solutions of the particular substance. In many instances, it is desirable to incorporate various solid materials, such as fillers, "into the fibers or filaments. However, it is not practical to do so when the material is to be extruded through fine orifices and the amount of foreign material which may be incorporated in the fiber or filament is strictly limited. In the production of sheet material or padding from such filamentary materials, it is necessary to collect the filaments and cut them to staple lengths. The staple is subsequently processed on conventional textile and Ifelting equipment to .form a woven sheet or felted layer or mat. Where the filament is formed of a potentially adhesive substance and vit is desired to produce a product, with or without non-adhesive fibers, wherein the fibers are bonded together at their points of contact, it is necessary to subject the sheet or mat, as formed, to an activating treatment whereby the potentially adhesive fibers become tacky or adhesive and are capable of bonding to other fibers at their points of contact and then deactivate the fibers.

As an alternative, .fibers or filaments may be formed from a wide range of substances by the use of various types of spray guns. Because of the relatively large-size of the spray gun orifices as compared to extrusion orifices such as those of a-rayon type spinneret, for example, the limitations with respect to the characteristics of the spinning liquid are far less exacting than for the extrusion methods. Various materials such as fillers, hardening agents, plasticizing agents and the like may be incorporated in the spinning liquids. If it is desired to form a product of potentially adhesive fibers and nonadhesive fibers, the potentially adhesive fibers may be produced by spraying into an air stream containing air-bornenonadhesive fibers and collecting the mixed fibers, for example as shown in the patent to Carleton 8. Francis, In, No. 2,357,392.

In the copending application of Howard 0. McMahon and Paul C. Watson, Serial No. 400,240, filed December 24, 1953, there is disclosed a method for the'production of reticulated fibrous webs of elastomeric material. In

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stenuates. the :plastic stream and breaks-the attenuated stream transversely to form the fibers; When certain thermoplast-containing solutions are employed as the spraying liquid, it is possible that the extruded stream of plastic .is also fibrillated ;by the *high'yelocity =.stream of v-gas,-.that is, the stream of-plastic is split lengthwise. Thegas stream at least partially removes the solvent or partially cools the molten plastic and, thereby partially lsets thefiber-forming material. Fibers of the partiallyset ffiberFforming-material are in an adhesive condition, :that is, they are ina sticky cementitious, agglutino'us,-or tacky condition and readily adhere to each other upon contact.

The .newly formed fibers amended by the gasstream to a suitable collecting surface or a secondary streambf gas may be'employed :to float-or carry the 'rfibers to the collecting surface. While the fibers-are thus-carried, they come into contact with each -other ;particularly ;in the case of :the fibers formed ofelastomeric materials :and the fibers may become intertwined to :form nope-like or string-like fibers of appreciable .size :wit'hout'actually The reticulated 'mat or sheetwforme'tl by collecting the fibers consist of fibers or fiber-like lbodies whose transverse or cross-sectional I dimensions *vary rover a considerable range.

One of the purposes of the present invention :is. to :pro-

v-ide an improved method for the production of fibers .andreticulated fibrous products.

.paratus .for the production of reticulated fib'rous products.

.A specific purpose 10f the present "invention is to provide a method and apparatus for the production of fibers by ;a spraying technique wherein the intertwine or roping of the fibers is substantially eliminated.

Other objects and advantages 'of this invention will become apparent from the :disclosure and :claims which follow.

-In the drawings, 1

Figure 1 is a diagrammatical elevational view partially in section of one form ofiapparatus for the preparation of reticulated fibrous products.

- Figure 2 is a diagrammatical elevational view partly in section of a further form of apparatus for :the products of this invention.

The present invention contemplates inducing an 'e'le'c-' tri'cal charge on :fi'bers immediately upon their being formedby the spraying techniques disclosed in the aforementioned applications. The fibers are formed by extrading a fiber-forming spraying liquid-as :a relatively large diameter stream of plastic into a high 'velocity stream of .gas and under the influence of a high potential electrostatic :field. The stream of plastic is "charged electrically, attenuated and broken transversely and may be fibrillate'd into a plurality of fibers within or under the influence pf .the electrostatic field. The 'fib'er's :are then collected upon a collecting surface which carries an elecby attract the fibers.

-Unlike dry'spinning, as performed in the rayon andsynthetic fiber industries, in which a spinneret with multiple minute holes is used to produce a predetermined number of filaments each substantially of the same size as the holes and in which the continuous filaments are pulled continuously from the face of the spinneret to a moving collector, the spraying process of the invention utilizes a single relatively large extrusion orifice producing a single large-diameter plastic stream whichis attenuated and possibly fibrillate'd into a multiplicity of fibers and fibrils, the diameters of which fibers and fibrils are small fractions of the orifice diameter, there being no continuous filament running between the orifice and the collector. q Y

The velocityof the gas into which the spraying liquid is extruded is appreciably higher than the velocity of extrusion of the spraying liquid and the direction of extrusion is coincident with the'direction of the gas flow. The high velocity stream of gas thereby attenuates and possibly fibrillates the spraying liquid, that is, thestream of liquid is highly attenuated and may be split lengthwise into two or more thinner fibers or fibrils and is broken up into discontinuous fibers or fibrils of varying length by the high velocity gas.

Immediately upon extrusion of the spraying liquid and the on-set of fiber formation an electrostatic charge is induced on the stream of plastic and on the fibers. This charge aids in fibrillation of the stream of plastic. Each of the fibers thus formed carries a like charge and the individual fibers thereby repeleach other.

The stream of gas partially or substantially completely sets the fiber-forming material due to a partial or substantially complete vaporization of the solvent or to a cooling of the spraying liquid to form the electrically charged,

' discontinuous, attenuated fibers or fibrils. The gas also carries, supports or floats the fibers or fibrils and finally deposits them or allows'them to deposit upon a suitable -collecting surface. Alternatively, a low velocity or secondary stream of gas may be provided having a velocity lower than that of the high velocity or primary stream of gas but having a sufiiciently high velocity so as to carry,

support or float the fibers and fibrils and finally deposit them or. allow them to'deposit upon a suitable collecting surface. The collecting surface may carry a charge opposite that of the charge induced upon the fibers to there- Alternatively, a charged electrode may be positioned behind a collecting surface so that the fibers are arrested in their travel toward the attracting electrode and will be collected in a charged form. The fibers may be collected in either a substantially set or.dry non-adhesive condition whereby the product is a loose fiufiy mat or web or the fibers may be collected in an adhesive or tacky condition whereby they become 7 bonded together at their points of contact to provide a coherent reticulated web or sheet. 7 v

Our invention is applicable to the production of fibers Y and fibrous mats or webs from rubber, both natural rubbers and synthetic rubbers or rubber substitutes. Such elastomeric materials or rubbers, both natural and synthetic, which are soluble in inexpensive, volatile organic --solvents are well suited for the production of the reticulated mats or webs of this invention. Elastomric mate- .rials satisfactory for use in the herein described method include natural rubbers such as crepe rubber and syn thetic rubbers or rubber substitutes such as chloroprene .polymers, for example, neoprenes; butadiene-acryloni- 'trile copolymers known as Buna-N, for example, Butaprene, Paracril, Ameripol-D, Perbunan, Chemigum, and

v and to reduce the cost of the fibers.

copolymers, for example, GR-I and butyl; and organic polysulfides, for example, Thiokol. Mixtures of specific elastomeric materials may be utilized to provide desired characteristics. The specific elastomers are enumerated merely as illustrative and are not intended as limitations of the invention.

The spraying or fiber-forming liquid may be formed by dissolving the fiber-forming elastomeric material in a satisfactory organic solvent such as aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, aralkyl hydrocarbons and the like, those being preferred which will volatilize readily at moderately elevated temperatures. The solvent utilized in forming the spraying liquid will be dependent upon the specific elastomer and upon characteristics desired in the spraying liquid such as volatility of the solvent. For example, solvents which are satisfactory include benzene, naphtha, toluene, xylene, cyclohexanone, ethylene, chloride, methylene chloride, carbon tetrachloride, nitroparaflins, ketones and the like.

,, Such inexpensive volatile organic solvents as benzene and naphtha are entirely satisfactory for use in fiber-forming liquids containing natural rubber. The spraying liquids may contain from about 5% to about 50% of the fiberforming' elastomeric material and preferably contain between about 10% and about 35% rubber or rubber substitute.

Gas-forming or'blowing agents such, for example, as ammonium carbonate, sodium acid carbonate, diazoaminobenzeue and the like, may be added to the elastomeric materials or spraying liquids, if desired. These agents include solids and gases and are commonly employed in the production of sponge rubber and porous rubber sheet and products. They are adapted to release or form a gas such as ammonia, carbon dioxide or other inert gas at a temperature at which the elastomeric material is cured or vulcanized.

The properties and characteristics of the fibers formed from the elastomeric materials may be varied as desired 'or vulcanizing agents'such as sulfur, accelerator, antioxidants, plasticizers and the like. ,Detackifying agents,

such as parafiin wax, stearic acid and the like may be incorporated in the spraying liquid so as to decrease the natural adhesive or'tacky nature of the unvulcanized fibers. Coloring agents, such as dyes and pigments may be utilized to produce fibers having desired colors or tints. .Abrasive particles such as emery dust, Carborundum, silica, etc., may be incorporated in the spraying liquid to provide products having abrasive properties. Fillers such as clay,.whiting, kaolin, French chalk and the like may be added to impart desired characteristics The amount of the additive may be varied over a wide range as desired. In the case of solid fillers, from about 50% to about filler, such as clay, finely divided pigments and the like, based upon the weight of the elastomer, may be incorporated in the spraying liquid. Lesser or greater amounts, however, may be employed depending upon the type of product desired.

The additive substances may be mixed with the elastomer as by milling the elastomer and the additive, or the additive substance may be mixed with or dispersed in the solution of the elastomer.

By varying the amount of solvent and the amount of additive substances and the of this invention which are totally unsuited for use in the usual or conventional spinning methods.

The'primary gas stream may be at normal atmospheric temperatures or any other desired temperatures. For

i'rample, the temperature may be elevated so :as to in- :orease'the rate of volatilization :of :the solvent. The :gas may consist :of :a chemically reactive gas, steam, air or other'inert gassuch as nitrogemcarbon dioxideand the like. Since the fibers as theyare formed by the attenuating efiect of the gas stream and the volatilization of the solvent are tacky-or cementitious, solid particles or short preformed fibers may be introduced into the'primary gas streamso as to provide a coating on the fibers. For example, a deta'ckif-ying agent such as talc, wood flour, starch, etc., may beintroduced to provide a coating which .decreases-ordestroys the natural tackiness of the fibers and allows the collection of the fibers inza lbody wherein :there is little or no :adhesion between :the crossing lilaiments. Short, preformed fibers either synthetic or artitficial such as yiscose rayon staple, cotton, wool, asbestos, -etc., or ifinely divided particles such as leather, .rayon flock, cork dustand'the likemaybe introduced if desired.

The invention .is also applicable to the production .of :fibers and reticulated fibrous webs or mats from :fiber- :forming, non-elastomeric thermoplast 'which are primarily .synthetic, nou-elastomeric polymers capable of forming fibers :from volatile organic solventsolutions or .iromhot melts or both, and are thermoplastic or may exist in a'thermoplastic state. The-spraying liquids comvprisedof the solvent solutions of .thessynthetic polymers or the hot melts of the polymers :or ;both :are capable of being attenuated .and fibrillated .to form the fibers. The *fihersof the polymer are capable .of being renderedad- .:hes'ive, sticky, .cementitious, agglutinous .ortacky by heating to temperatures elevated with respect to :normal or room temperatures :or by treatment with a solvent or :hoth, and the polymers andrfibers formed therefrom may nherefore lbe termed potentially adhesive materials? The non-elastomeric thermoplasts satisfactory for the purposes of this invention consist of a wide variety of substances selected generally from the classes orgroups consisting of organic cellulose derivatives, thermoplastic resins and thermosetting resins in their thermoplastic state. These 'thermoplasts include organic solvent soluble cellulose esters such as cellulose acetate, cellulose acetate-butyrate .and the like; organic solvent soluble cellulose ethers such as ethyl cellulose, and the like; vinyl resins, which may be defined as solid, thermoplastic, saturated synthetic resins resulting from the polymerization of compounds containing the vinyl group such "as .polyvinylchloride, copolymers of vinyl chloride and vinyl :acetate (Vinyon), copolymers of vinyl chloride and acrylonitrile ;(IDynel), polyacrylonitrile including copolymers =of polyacrylonitrile containing a predominant proportion of acrylonitrile (Acrilan, Orlon), polystyrene, :polyacrylates such as ethyl acrylate polymers, methyl acrylaterpolymers and the like; polyesters (Dacron); polyamides (nylon), including alcohol soluble nylon; organic solvent soluble thermosetting resins of the types which are capable of existing in a thermoplastic state :such, for example, as urea-formaldehyde, melamine-formaldehyde, thiourea-formaldehyde, glyptals, silicones and the like, which resins are attenuated and fibrillated while they are in their thermoplastic state and which, if desired, may be converted subsequently to the thermoset state while in fiber form; and the like. Mixtures of specific thermoplasts may be utilized to provide fibers and structures of desired characteristics. The specific thermoplasts are enumerated merely as illustrative and are not intended as limitations of the invention.

The spraying liquid may be formed by dissolving the fiber-forming thermoplast in a suitable organic solvent which may be volatilized at moderately elevated temperatures, or .thespraying liquid may comprise a hot'melt or :molten mass of the thermoplast. For example, a spraying liquid may comprise a solution of Vinyon in tetrahydrofurane and methyl isobutyl ketone, or an alcohol solution of alcohol soluble nylon, or a hot melt of nylon. Where the spraying liquid comprises a solution of a thermoplast, the solution may contain from about 10% :to about 'ofrthefiber-forming thermoplast, the specific'solids content varying with the difierent types of fiber-forming materials.

The properties and characteristics of the fibers formed from the thermoplasts may be varied .as desired by incorporating additives in the spraying liquid For example, fillers, t'coloring agentssuchas dyes and pigments, plasticizers .and the like may be incorporated in thesolution'of the thermoplast or in the hot melt. By varying the relative proportions .of :solvent, thermoplast and additive substance, the viscosity of the spraying 'liquidmay be varied over .a wide range. This possible to utilize spraying liquids informing .theproducts of this invention which are 'totally:unsuited:for1use :in the usual .or conventional spinning methods.

The gas streams for attenuating the plastic stream and carrying the :fibers,.may be at normal atmospheric temperature .or at any other desired temperature. For example, the temperature may be elevated so as to increase ,the rate 'of yolatilization of the solvent. Any desired :gasumay be employed such i-asair, :nitrogen, carbondioxide, steam and the like.

The elastomers :and :thermoplasts are potentially .ad- .hesiye materials, as described hereinbefore. Since the fibers as they are fformed are adhesive or;tack.y,.a reticulated .web or structure can be :produced by collecting them :on .a surface, or a composite reticulated web or structure :can be produced by collecting .them in combination with other particulate material such as preformed fibers .and/or discrete .particles, preferably of a non-adhesive, .non-elastomeric material, which may be introduced into the streamsxzof gas. The preformed fibers or discrete .particles 'are thereby brought into contact with the potentially adhesive fibers while the latter are in a tacky or ce-ment'itious condition and the preformed particulate material adheres to the newly formed fibers. The fibers may be collected while they are in an adhesive condition .or afterwards. The fibers as collected are deposited in a completely random distribution or haphazard manner .to produce a reticulated, Web or structure. Where the fibers are collected while they are in a tacky condition, the fibers will stick together where they contact each other and 'where they contact preformed particulate material, if such :material is present. The fibers maybe collected after .complete removal of the-solvent or a cooling of the fibers to a temperature at which they are not tacky in which case they will not stick together. the reticulated web may be thus controlled as desired. Conventional methods of handling staple fibers, even though elaborate carding equipment is employed, do not produce webs having the totally random distribution of fibers as formed by this method. This .method also eliminates the activating procedure required where conventional methods are utilized in forming the mixed fiber or composite bodies.

The term particulate non-adhesive material is used to designate preformed fibers and/or discrete particles of a material which is not rendered adhesive or tacky under the conditions at which the elastomers and thermoplasts are adhesive.

Among the non-adhesive substances or materials which may be employed in forming the composite bodies are natural fibers, such for example as wood fibers, cotton, flax, jute, sisal, kapok, wool, hair, and silk, other natural substances such as leather and cork; synthetic fibers, for example, cellulosic fibers such as cellulose hydrate, cellulose derivatives such as cellulose esters, mixed cellulose esters, cellulose ether-s, mixed cellulose ester-ethers, mixed cellulose ethers, cellulose hydroxyalkyl ethers, cellulose carboxyalkyl ethers, cellulose etherxanthates, cellulose Xantho-fatty acids, cellulose thiou rethanes, fibers made of alginic acid, gelatine, casein; mineral fibers such as spun glass, asbestos, mineralwool The degree of bonding of .the fibers in and the like; and fibers made of natural and synthetic resins which are not rendered tacky when the potentially adhesive thermoplast fibers or fibrils are rendered tacky; also fibers and filaments made by slitting, cutting or shredding non-fibrous films, such as waste cellophane.

In addition to or in lieu of the preformed fibers, preformed discrete particles of natural and synthetic materials may be introduced into one or both of the streams of gas; for example, cork dust, leather dust, wood flour, flake particles, fibers of floc length and the like. Two or more different non-adhesive materials may be introduced into the gas streams and the different materials may be of different physical form; for example, one may be in fiber form and another in powdered form, depending upon the nature and characteristics desired in the final product. In producing composite webs or structures including preformed non-adhesive fibers, it is preferable to employ both theprimary and secondary streams of gas and introduce the preformed fibers into the secondary air stream. Preformed discrete particles may be introduced into either gas stream. Where the composite body is to include both preformed fibers and discrete particles, the discrete particles, for example, cork dust or wood flour, may be introduced into the primary stream of gas and the preformed fibers, for example, wood fibers, rayon staple or mineral wool fibers, may be introduced into the secondary stream'of gas. Both types of preformed non-adhesive material, however, may -be introduced into the primary or high velocity stream of gas or into the secondary stream of gas.

The method of the present invention may be practiced by the use of apparatus as illustrated more or less diagrammatically in Figure 1. A tower or chamber 1 open at its upper end is provided with means such as a blower 2 for passing a gas at a relatively low velocity through the tower. At or adjacent the base of the tower,

there is mounted a spraying unit 3 which comprises a spraying liquid or spray tube 4 having an extrusion orifice at its upper end, the other end communicating with a liquid conduit 5 through which the spraying liquid is passed under pressure. Surrounding the spraying liquid tube 4 there is positioned a gas nozzle 6, the spray tube 4 extending beyond the end of the gas nozzle so as to position the extrusion orifice in the path of the gas issuing from the nozzle. The gas nozzle is connected with suitable means such as a blower 7 for passing a gas through the nozzle at a relatively high velocity. The

. spraying unit is preferably mounted concentrically with respect to the walls of the tower or chamber. A collecting surface such as a conveyor screen 8 is positioned across the open end of the tower. An open ended cylinder 9 is mounted in spaced relationship with respect to the spinning unit 3 as by means of insulation 10. Sources of high potential 11 and 11a such as generators having .a capacity of from about 5,000 volts to about 50,000

volts are connected electrically at one side to ground. The spraying unit 3 and tower 1 are also grounded. The cylinder 9 and the conveyor screen 8 are independently connected to the other side of the high potential sources.

The spraying liquid is extruded into the high velocity stream of gas in the form of a single relatively large diameter stream of plastic. The high velocity stream of gas draws or attenuates the spraying liquid and breaks the stream transversely into discontinuous lengths to form a plurality of fibers of varying length. In some intion. Solvent is evaporated simultaneously .v'vith-the attenuation action so as to partially set or hardenthe fiber-forming material or if the spraying liquid comprises a hot melt, the partial setting or hardening isaccomplished by a partial cooling of the attenuated stream.'i;

Simultaneously, with the extrusion and with .the filament formation under the influence of the electrostatic field, an electrical charge is induced upon the filaments, the charge being opposite that applied to the charging cylinder 9. Since all of thefibers thereby carry, the same induced charge, they will repel each other and prevent or eliminate intertwining or roping of the fibers during their travel in the air stream.

The charged fibers or fibrils are carried or floated in the gas stream and are also carried towards the target by the attractive forces produced by the difference. in potential between the fibers and the collecting surface, which may be grounded or preferably carries a charge opposite to that of the fibers. Solvent continues to volatilize from the fibers or the cooling of the molten fibers continues during the passage of the fibers'to the collecting surface where the fibers are finally deposited or collected. As the fibers collect on the screen or other porous or foraminous surface, the resistance of the reticulated web or mat 12 to the passage of gas increases and for the production of thicker webs or mats, a suction chest 13 may be positioned on the opposite side of the porous collecting surface or screen. The suction chest may also serve as an aid to the recovery of vaporized solvent, if desired. In some instances, particularly in the production of reticulated webs and mats of the elastomeric materials, it is desirable to utilize a secondary stream of gas which may be supplied by the blower 2 and which surrounds or envelops the primary. stream'of gas. The secondary stream of gas is passed through the chamber or tower at a lower velocity than the primary or high velocity stream'of gas and serves to carry or float the fibers to the collecting surface and also complete the removal of solvent and the setting of the fibers. The gas streams may be at' any desired temperature so as to regulate the condition ofv the fibers at the time of collection on the collecting surface. By increasing the gas temperatures, the fibers may be deposited in a substantially non-adhesive condition.

The characteristics of the reticulated mat or web may be varied by altering the positioning of the collecting surface with respect to the spraying unit. By decreasing the distance between the collecting surface and the spraying unit, the distance through which the fibers travel before being collected is also decreased. The fibers have had a shorter period to become set and are accordingly deposited in a more tacky condition to form a more thoroughly bonded mass. The velocity of the fibers at the instant of deposition is higher the shorter the distance of travel and, hence, the mat or web will be of higher density. By increasing the distance between the collecting surface and the spraying unit, the fibers will be in a less tacky condition and will be deposited at a lower velocity and the web or mat is lower in density and the fibers are less firmly bonded together.

In production of fibrous webs or mats utilizing the present method, the collecting surface or screen may be spaced from about 3 feet to 10 or 12 feet from the spraying unit depending upon the film-forming material, the specific spraying liquid, the spraying conditions, and the type of product desired.

The reticulated web or mat may be passed through a heating chamber 14 before being removed from the collector screen by a doctor blade or dolfer 15. The heating chamber 14 may be employed to activate the fibers to provide a greater bond between them or in the case of ela'stomeric fibers the heating chamber may be employed to effect a curing or vulcanization of the elastomer. For the production of thin sheet-like products, the reticulated web upon activation of the fibers may be passed between pressure rolls T6 or the pressure rolls may be provided with gem-42s 9 desired protuberances to provide additional bonding at predetermined localized .areas. The heater may .be replaced by heated pressure rolls. The reticulated web is finally accumulated on the take-up roll or drum 17.

In the production of fibrous bodies comprising pre- 1 and thereby become firmly attached to the sprayed fibers.

The spraying conditions are maintained so that the sprayed fibers are still in a somewhat tacky condition ing units may be mounted within a large tower.

As illustratedin Figure 2, the tower 1, the spraying unit 3 and one side of the high potential source-11 are connected electrically to ground. The'charging-electrode or cylinder 9 and a second electrode 18 are connected electrically to the other sides of independent high potential sources. The fibers may be collected upon a-porous or foraminous collecting surface 19 such as a porous flexible conveyor belt formed of an electrical insulating material which is interposed in the path of the fibers between the spraying unit 3 and charging electrode 9 and the second electrode 18 which may comprise a metallic plate. The collection of the fibers on the porous collecitor surface occurs as thefibers are arrestedin their flow toward itheelectrode :18.

Although the apparatus as illustrated shows the spraying operation .as being performed by establishing an up wardly..directed'ahigh velocity stream of gas and extruding the rfibereforming liquid upwardly into the stream of gas. Equally satisfactory results and products may be produced by spraying downwardly or horizontally or'in any other desired direction.

The spraying and fiber-forming method has been found to be particularly satisfactory and inexpensive for the production .ofreticulate webs and mats consisting of very fine fibers. Spraying liquids having a viscosity or apparent viscosity within a range of from about 150 C. P. S. to about 30,000 C. P. S. have been satisfactorily utilized in-preparing reticulate webs and mats. The viscosity and the apparent stringiness of the spraying liquids do not ap pear to be a measure of the fiber-forming characteristics when utilized in the method of this invention. The size of the fibers may be controlled by varying the concentration of the fiber-forming material in the spraying liquid, by varying the side of the extrusion orifice, by varying the relative velocity of the high velocity stream of gas with respect to the velocity of extrusion of the spraying liquid and to some degree by varying the potentially applied charging electrode 9. As pointed out hereinbefore, the application of the charge to the fibers as they are formed substantially eliminates the intertwining and roping of the fibers during their travel to the collecting surface. We have also discovered that the fiber size is, in general, smaller than that of fibers produced without spraying under the influence of an electrostatic field and that the range of size is substantially reduced by the present method. Reticulate webs or mats prepared as described are more uniform and smooth in appearance than similar products prepared without inducing .the

electrical charge on the fibers.

It is well known that for certain types of gas filters it :is essential to employ fine fibers, that is, .fibers having a diameter of the order of 2 to 3 microns and lower.

1 rzConsiderableidifiiculty .is encountered Ein-preparihgfiltbt media from such ifine -rfibers because they cannot she handled :by conventional commercial =fiber :and textile -.equipment thus rendering such media-quite costly. The

zonly'iknown materials commercially :available are certain grades of glass :fibers .and :importedasbestos. 1 No comparable .organicfibers, either .natural or synthetic, (are available. Y

.Thespresent method provides :an inexpensivereticulated filtermediuni of very .finefibers which is admirably suited for both .gas andiliquid filters, gasmasks, and the like. The reticulated, sfibrous :bodies .are .also very highly e'f- 'fective 'as thermal :and .soundrinsulating materials. Becausle of .the relatively light .weight-of the webs or mats,

the bodies or structures are especially well suited for .such uses as .thermal .and sound insulation for aircraft, thermal insulation for arctic clothing and the like. :For example, reticulated webs, highly satisfactory for filter :purposes have been .prepared ,:for spraying liquids vcontaining Ninyon, polyacrylonitrile and Dynel. The fibers zhavexhad :aidiameterof .lessi'than 1 micron to about 3 microns with :the'tlarge majority of the fibers having a .diameter within therange .of. '1 to 13 microns.

' ilnassmall scale production of fibrous, reticulated mats :or webs of this invention, 2a tower may be employed .1ha.ving:a diameter ;of four feet and aheight of about twenty-five :feet. The spraying unit 3 consisting of a spinning tip .4 and a nozzle 6 is mounted concentrically .within the tower .ata point from about 3 feet to about 15 .feet from .-the.collecting screen. If the filament-forming material is to be extruded-downwardly, the spraying unit may :be mounted adjacent or at the top of the tower. Aipluralityxofspaced spraying units may be employed if .desired.

'A :blower :may' be provided to introduce an "inert gas such as air at :theibottom of the tower andsuitable heating .means maybe provided topermit regulation-of the temperature .of the air stream if a-secondary stream of tgaslis employed. The collecting conveyor may consist .o'f'a-suitable screen such as :a screen formed of Saran. .The spray or extrusion-orifice for the :thermop'lasts may be varied. Orifices between about 0.01 inch and about 0.05 .inch in diameter :have been satisfactory. The -primarygas stream or high velocity gas" stream may be provided through a. nozzle havingan internal diameter of from about 0.25 inch to about 0.75 inch. "The-extrusion orifice is positioned-from about 0.25 inch '=to 1 inch beyond the terminus of the nozzle. The thickness of the reticulated web or mat is controlled by regulating the speed of the collecting conveyor.

For example, reticulated fibrous webs have been produced in a tower as described above in which the .collecting screen was spaced about 4 feet from the spinning unit. The orifices through which the fiber-forming liquid was extruded, had a diameter of about 0.010 inch and the internal diameterof the gas nozzles was about 0.25 inch. Four spraying units were employed with a spacing of about 1.5 inches between adjacent extrusion orifices. The charging cylinder and collecting screen were maintained at a positive potential with respect to the spraying unit of about 20,000 volts. For example, a spraying liquid containing about 25% Vinyon in tetrahydrofurane was extrudedata velocity of about feet per minute and air supplied "to the gas nozzle at from 24,000 to 27,000 feet per minute. The reticulated web was formed by collecting .the fibers without the use of a secondarystream :of gas. The large proportion of the fibers .hadladi- 'ameter in the vicinity of .1 micron and microscopic .examination showed thatthe fibers were relatively straight and within the range of a fraction of a micron to about 3 microns in diameter. Thecollected web was light in weight and fluffy-with the fibers loosely bonded. together andhad :a very smooth and warm feel.

In a similar ;manner, webs were produced :by using a 21% solution of Dynel in dimethylformamide.

sfibersize was within. the same range-and like the Vinyon afibers, the fibers were relatively straight and" uniform. :1 Fibrous. webs have also been prepared from spraying ;liquids "containing from to about polyacrylonitrile dissolved in dimethylformamide. -fibers are very regular and smooth and appear to be of about the'same diameter, namely about '1 micron.

In the production of similar products from elastomeric materials, ,the collecting surface is preferably spaced at least about 5 or 6'feet. from thespraying'unit. Ex-" trusion orifices may vary in diameter depending upon the fiber size desired and the gas nozzle diameters may also be varied. We have employed satisfactorily spray- .ing tubes having orifices up to about 0.06 inch in diameter and gas nozzles having diameters of up to about 0.75 inch., The extrusion orifice may extend from about 0.25 inch to about 1 inch beyond the terminus of the nozzle. It is also preferable to employ a secondary air stream because of the greater density of the individual fibers. The elastomeric materials may be combined with anti-oxidants, fillers, accelerators and coloring agents before dissolving or suspending in benzene or naphtha base solvents. Satisfactory products have been prepared from natural rubber, for example, pale crepe, and synthetic rubbers or rubber substitutes, for example, neoprene, by extruding the spraying liquids at a velocity from about 50 feet per minute to about 100 feet per minute. The primary or high velocity stream of air had a velocity of from about 20,000 to about 40,000 feet per minute. Secondary air velocities have been varied from about 500 to about 700 feet per minute. i

The webs and mats formed by inducting an electrical charge on the plastic stream and the fibers have been more uniform in appearance and there is substantially no intertwining or roping of fibers apparent in the products. The fiber size is much more uniform as compared to similar products prepared without inducing a charge on the fibers as they are formed. In general, the fibers are of smaller diameters for the same spraying conditions as well as being more uniform when compared with fibers prepared without the influence of the electrostatic field.

Filters formed of the reticulated webs or mats of thermoplasts have exhibited filtration properties which are as satisfactory and in some cases of higher efiiciency than thehighest grade, commercially available filter materials formed of glass and mineral fibers. In general, these filter bodies have also exhibited an appreciably lower density than glass and mineral fiber filters. For example, a web of Vinyon fibers prepared as described herein having a density of 6 grams per square feet has substantially the same filtration efiiciency as a Web or mat of glass fibers having a density of 10 grams per square feet.

The following table is a representative comparison of filter sheets prepared in accordance with this invention .and of two commercially available filter mediums when used to filter a 0.3 micron smoke:

Filter bodies of various thicknesses and density have been prepared and their efficiencies compared with the AEC absolute filter when using a 0.3 micron smoke (dioctyl phthalate vapor). The results of these tests are set forth in the following table:

The individual Table II Thick- Density Velocity Pressure :ness (gm/sq. of gas drop Percent v (inches) ft.) flow (mm. efficiency (ft/min.) H2O) v AEG Absolute Filter 20. 100 99.9 Dynel..- 0.024 8.9 28 59 97.5 Dyne 0.041 p 15. 0 28 103 99. 6 Polyacrylonltrlle 0.020 7.5 28 40 92.0

- The products of this invention are not limited to the use as gas filters but may be employed satisfactorily for any applications requiring low density, fibrous structures such as for the acoustical treatment of aircraft or similar applications where the weight of the sound attenuating or absorbing material must be kept to a minimum.

Obviously, the fibrous products are also highly satisfactory for applications where the weight need not be held at a minimum. The fibrous products are also highly efficient when utilized as thermal insulation as in aircraft, arctic clothing, and the like. The fibers may be employed for other purposes by collecting the fibers in a non-adand the like.

hesive state so that the individual fibers may be readily separated from the collected mat or web. Such fibers may be employed in the preparation of the textile yarns The products prepared from the elastomeric materials may be employed for a wide variety of uses such as the preparation of elastic fabrics for the manufacture .of

. wearing apparel such as foundation garments, bathing suits and the like. For these purposes the productsimay be employed either in the form of a reticulated, fibrous rubber web or sheet or in the form of composite structures wherein the fibrous web is combined with a textile material. The fibrous web of the elastomeric material may also be utilized as a fibrous web or in the form of a composite structure for use as belting,-galluses,'garters and the like. The fibrous webs of elastonieric materials may be secured to or combined with sponge or foam rubber sheets to reinforce the foam or sponge rubber.

Other uses will occur to those skilled in the art.

. which comprises providing a fiber-forming liquid containing a potentially adhesive, fiber-forming organic material .selected from the group consisting of thermoplasts and elastomeric materials; establishing an electrostatic field of high potential; establishing a high velocity stream of gas; discharging the stream of gas into the ambient atmosphene'within the electrostatic field; extruding the fiberforming liquid intovand within the stream of gas at a point beyond the point of discharge of the stream of gas and under the influence of the electrostatic field to induce an electrical charge on the extruded liquid, the direction of extrusion being coincident with the direction of the gas flow; attenuating the extruded fiber-forming liquid, breaking the attenuated fiber-forming liquid into discontinuous lengths and at least partially setting the fiber-forming liquid to form discontinuous, electrically charged fibers by maintainingthe velocity of the stream of gas at the point of discharge at'a value greater than the velocity of extrusion of the fiber-forming liquid; and collecting the discontinuous fibers in random distribution to form a reticulated, sheet-like fibrous body.

l 2. The method as defined in claim 1, wherein the fiberforming liquid comprises a solution of a thermoplast.

3. The method as defined in claim 1 wherein the fiberforming liquid comprises a molten thermoplast.

4. The method as defined in claim 1 wherein the fiber forming liquid comprises a solution of a vinyl resin.

5. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of a copolymer of vinyl chloride and vinyl acetate.

6. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of a copolymer of vinyl chloride and acrylonitrile.

7. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of a polyacrylonitrile.

8. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of an elastomeric material.

9. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of a natural rubber.

10. The method as defined in claim 1 wherein the fiberforming liquid comprises a solution of a synthetic rubber.

11. The method as defined in claim 1 wherein the fibers are collected on a surface having an electrical charge opposite the electrical charge carried by the fibers.

12. The method of producing a reticulated, fibrous body which comprises providing a fiber-forming liquid containing a potentially adhesive, fiber-forming organic material selected from the group consisting of thermoplasts and elastomeric materials; establishing an electrostatic field of high potential; establishing a primary high velocity stream of gas; discharging the stream of gas into the ambient atmosphere within the electrostatic field; extruding the fiber-forming liquid into and within the primary stream of gas at a point beyond the point of discharge of the stream of gas and under the influence of the electrostatic field to induce an electrical charge on the extruded liquid, the direction of extrusion being coincident with the direction of the gas flow; attenuating the extruded fiber-forming liquid, breaking the attenuated fiber-forming liquid into discontinuous lengths and at least partially setting the fiber-forming liquid to form discontinuous, electrically charged fibers by maintaining the velocity of the primary stream of gas at the point of discharge at a value greater than the velocity of extrusion of the fiber-forming liquid; and suspending the fibers, completing the setting thereof and depositing the charged fibers in random distribution to form a reticulated, sheetlike, fibrous body by passing the ambient atmosphere upwardly at a velocity greater than the velocity of extrusion of the fiber-forming liquid but lower than the velocity of the primary stream of gas.

13. The method as defined in claim 12 wherein the fiber-forming liquid comprises a solution of a thermoplast.

14. The method as defined in claim 12 wherein the fiber-forming liquid comprises a solution of an elastomeric material.

15. The method as defined in claim 12 wherein the fibers are deposited on a surface having an electrical charge opposite the electrical charge carried by the fibers.

16. Apparatus for the production of permeable, reticulated, fibrous bodies of fiber-forming organic materials which comprises a chamber; means including a gas nozzle for passing a stream of gas at a high velocity through the chamber; means including a spraying tube terminating at a point beyond the end of the gas nozzle for extruding a spraying liquid containing a fiber-forming organic material into and within the high velocity stream of gas at a point beyond the end of the gas nozzle thereby attenuating the extruded liquid and breaking the extruded liquid to form discontinuous fibers; means including a source of high potential and an electrode connected electrically to one side of the high potential source and positioned in close proximity to the gas nozzle and the spraying liquid tube for inducing an electrical charge on the extruded fiber-forming liquid and the fibers; and means for collecting the charged fibers.

17. Apparatus for the production of permeable, reticulated fibrous bodies as defined in claim 16 wherein the means for collecting the fibers comprises an electrode connected to the same side of the high potential source.

18. Apparatus for the production of permeable, reticulated, fibrous bodies of fiber-forming organic materials which comprises a chamber; means including a gas nozzle for passing a stream of gas at a high velocity through the chamber; means including a spraying tube terminating at a point beyond the end of the gas nozzle for extruding a spraying liquid containing a fiber-forming organic material into and within the high velocity stream of gas at a point beyond the end of the gas nozzle thereby attenuating the extruded liquid and breaking the extruded liquid to form discontinuous fibers; means including a source of high potential and two electrodes connected electrically to one side of the high potential source, one of the electrodes being positioned in close proximity to the gas nozzle and the spraying tube for inducing an electrical charge on the extruded fiber-forming liquid and ,the fibers, and the other of the electrodes being positioned remotely from the gas nozzle and the spraying liquid tube in the path of the stream of gas; and means for collecting the charged fibers.

19. Apparatus for the production of permeable, reticulated, fibrous bodies as defined in claim 18 wherein the first, named electrode consists of an open ended cylinder and the second named electrode consists of a porous, conveyor belt.

20. Apparatus for the production of permeable, reticulated, fibrous bodies as defined in claim 18 wherein the first named electrode consists of an open ended cylinder and the means for collecting the charged fibers comprises a porous, collecting surface interposed between the two electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,336,745 Manning Dec. 14, 1943 FOREIGN PATENTS 376,598 Germany Dec. 21, 1921 495,737 Belgium Sept. 1, 1950 

18. APPARATUS FOR THE PRODUCTION OF PERMEABLE, RETICULATED, FIBROUS BODIES OF FIBER-FORMING ORGANIC MATERIALS WHICH COMPRISES A CHAMBER; MEANS INCLUDING A GAS NOZZLE FOR PASSING A STREAM OF GAS AT A HIGH VELOCITY THROUGH THE CHAMBER; MEANS INCLUDING A SPRAYING TUBE TERMINATING AT A POINT BEYOND THE END OF THE GAS NOZZEL FOR EXTRUDING A SPARYING LIQUID CONTAING A FIBER-FORMING ORGANIC MATERIAL INTO AND WITHIN THE HIGH VELOCITY STREAM OF GAS AT A POINT BEYOND THE END OF THE GAS NOZZEL THEREBY ATTENUATING THE EXTRUDE LIQUID AND BREAKING THE EXTRUDED LIQUID TO FORM DISCONTINUOUS FIBERS; MEANS INCLUDING A SOURCE OF HIGH POTENTIAL AND TWO ELECTRODES CONNECTED ELECTRICALLY TO ONE SIDE OF THE HIGH PONTENTIAL SOURCE, ONE OF THE ELECTRODES BEING POSITIONED IN CLOSE PROXIMITY TO THE GAS NOZZLE AND THE SPRAYING TUBE FOR INDUCING AN ELECTRICAL CHARGE ON THE EXTRUDED FIBER-FORMING LIQUID AND THE FIBERS, AND THE OTHER OF THE ELECTRODES BEING POSITIONED REMOTELY FROM THE GAS NOZZLE AND THE SPRAYING LIQUID TUBE IN THE PATH OF THE STREAM OF GAS; AND MEANS FOR COLLECTING THE CHARGED FIBERS. 